JPH07-239025A published by Japan Patent Office in 1995 discloses a lockup clutch which engages a turbine runner with a pump impeller of a torque converter for a vehicle in order to suppress increase in fuel consumption due to slip between the pump impeller and the turbine runner in a running region where a torque multiplication function or a shock absorbing function of the torque converter is not required,
The operation state of the torque converter where the turbine runner is engaged with the pump impeller is named a “lockup mode”. The operation state of the torque converter where the turbine runner is released from the pump impeller and torque is transmitted only via fluid in the torque converter is named a “converter mode”. The operation state of the torque converter where the turbine runner is partially engaged with the pump impeller and they transmit part of the torque therebetween in a slipping state is named a “slip mode”.
The switch-over of the operation modes is performed by varying a differential pressure between an application pressure and a release pressure both of which are supplied to the lockup clutch. Herein, the minimum differential pressure produces the converter mode, while the maximum differential pressure produces the lockup mode,
Selection of the operation mode is performed on the basis of a vehicle speed and a throttle opening of an internal combustion engine which drives the vehicle. When the operation mode is switched over from the converter mode to the lockup mode, the differential pressure is first increased by an open loop control. When a relative rotation speed between the pump impeller and the turbine runner (hereinafter referred to as a slip rotation speed) has decreased to a certain level as a result of this open loop control, the operation mode is switched over to the slip mode where the differential pressure is increased by feedback control and finally the lockup clutch is fully engaged.